The invention is directed to the recovery of data stored in magnetic storage media and, in particular, to an all digital method and apparatus for recovering non-return-to-zero (NRZ) information from modified frequency modulated (MFM) data from magnetic storage media.
In magnetic storage media, data are recorded in the form of magnetic flux transitions. Because flux transitions, rather than flux states, represent data states, and because in strings of the same data state no data transistions are present, various encoding techniques are used to assign flux transition sequences for encoding digital data. Among the advantages of using such encoding techniques include a self-clocking capability as well as greater data packing densities. One such encoding scheme is MFM encoding.
MFM encoding involves the use of the position of a pulse or transition within a bit cell to designate the meaning of the datum represented within each cell. There are a number of variations on the encoding scheme used to implement MFM coding. Among those is a scheme in which a one's bit is represented by a flux transition or pulse in the middle of a bit cell. A zero's bit is represented by a flux transition or pulse at the beginning of a bit cell or no flux transition throughout the bit cell, the latter occurring whenever the particular bit cell is preceeded by a bit cell having a one's bit represented therein. For example, a 10100 bit stream would be represented by a first cell having a flux transition at its midpoint, a second cell having no flux transitions throughout, a third cell having a flux transition at its midpoint, a fourth cell having no flux transitions throughout, and a fifth cell having a flux transition at its starting point. By encoding digital data in this manner the maximum gap between flux transitions is approximately two bit cells, while the spacing for long strings of bits having the same value will be equal to one bit cell. Packing densities are thereby increased.
In the typical magnetic storage media data recovery system several practical problems arise including, variations in the positions of the flux transitions due to variations in the media drive mechanism, and apparent flux transition position shifts when packing densities are very high. In the latter phenomenon, the flux magnitude of adjacent flux transitions tend to modify the flux magnitude of the transition of interest, so that the peak of each transition is effectively shifted in position away from its expected position within the bit cell.
Historically, analog phase locked loop techniques have been used to correct for the above drive variations and bit shifts. The phase locked loop systems included a phase detector, a filter, and a VCO, the VCO being implemented so that its period defined the window edges by which the data from the magnetic media were examined. The VCO frequency was typically two to four times the data rate. Analog phase locked loop systems were not without deficiencies, however. For example, because a phased lock loop has a natural frequency (rest frequency or center frequency) when out-of-lock an initial phase difference between the center frequency of the phase locked loop and the bit cell must be overcome before tracking of the signal can begin.
Additionally, the typical analog phase locked loop has a finite bandwidth. The magnitude of this bandwidth is chosen with several criteria in mind, including the ability to stably track data variations, and the ability to lock to the incoming data frequency quickly. In order to obtain the former, a narrow bandwidth is required. However, this narrow bandwidth limits the speed with which the loop can obtain lock. On the other hand, a wide bandwidth permits the phase locked loop to obtain phase lock quickly. A wide filter bandwidth, however, causes the phase locked loop to be susceptible to noise in the incoming data stream.
One attempt to correct for such problems involves the use of two filters having different bandwidths, one for acquisition purposes having a wide bandwidth, and another for tracking purposes having a narrow bandwidth. However, the use of such dual mode phase locked loops leads to complex circuit and alignment requirements as well as other problems.
Several digital apparatus for recovering MFM encoded data exist. Frazier, U.S. Pat. No. 4,222,080 is perhaps the most pertinent. In Frazier, the periods between pulses are classified as long, middle, and short. Ratios of the number of short pulses, middle pulses, and long pulses with respect to one another are formed in order to determine whether a particular pulse represents a one or a zero data bit. As implemented, it appears that the circuitry in Frazier required to handle high bit rates and packing densities would be impracticable to build.